Vehicle controllers are typically powered by a power supply which offers a dual voltage output. The higher of the two voltage commonly powers the input/output (I/O) ring of the controller which comprises logic gates requiring a standard voltage level. The lower voltage level powers the internal bus logic of the digital core. The lower voltage output is becoming a necessity because of the increasing speed and lower power target of modern microprocessors. The source of the lower voltage output typically contains a current limiting and current foldback feature which controls and reduces the amount of power dissipation during a short circuit.
At a power on event, the higher and lower voltage gradually ramp up to their designed voltage level. The higher voltage is generated directly from a switching power supply, whereas the lower voltage results from feeding the higher back through a linear voltage regulator. Therefore, during the voltage ramp up, the controller monitors the ramping of the higher voltage and begins to ramp up the lower voltage at a designated point. This designated point is a function of the differential which must be maintained between the higher and lower voltage.
The vehicle controller power supply protects itself in the event of a short circuit by having a current foldback feature. Both the higher and lower voltage sources are designed to handle a specified operating current. If the transient current load during power up is higher than the maximum normal operating current of the lower voltage source, the foldback feature will never allow it to reach its designated voltage value. As a result, the specified differential between the higher and lower voltage is violated. Typically the short circuit current value is 15-20% of the maximum steady -state current. By the foldback feature limiting the current to this value, the lower voltage will not achieve its steady -state.
It is an object of the present invention to provide a circuit which prevents the lower voltage source from being held by the foldback feature in response to large transient demands during power up.
This invention is directed to a power up assist circuit for use with a multiple voltage output power supply. The power up assist circuit includes a first circuit loop. The first circuit loop has a first voltage output. A second circuit loop has a second voltage output. During an initial power up stage, the first circuit loop supplies the second voltage output so that the second voltage output is assisted by the first voltage output through the first circuit loop. The second circuit loop deactivates the first circuit loop after the second voltage output achieves a predetermined value so that the first circuit loop no longer supplies said second voltage output.
Implementation of the assist circuit of the present invention does not allow the higher and lower voltages to violate the specified difference between the two. Therefore, the controller remains operable, not going into a foldback state upon a large transient demand.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.